MUC1 (CD227) is a type I membrane glycoprotein composed of heterodimers of a large N-terminal subunit (MUC1-N) covalently bound to a small C-terminal subunit (MUC1-C).
The N-terminal subunit (MUC1-N) is the large extracellular domain, which consists of the variable number of tandem repeats region (VNTR) and the non-VNTR region. MUC1-N is shed from the cells and can be found in the circulation of patients with advanced cancer. MUC1-N is used as a tumor marker (CA15.3) in breast cancer patients (see Hayes et al., J. Clin. Oncol., 4: 1542-50 (1986)).
The C-terminal region of MUC1 (MUC1-C) has three distinctive parts: a small extracellular domain that is covalently bound to MUC1-N, a single transmembrane domain, and a cytoplasmic tail (see Lan et al., J. Biol. Chem., 265: 15294-9 (1990)). The cytoplasmic tail contains sites for interaction with signaling proteins, such as β-catenin, epidermal growth factor receptor (EGFR), and Src (see Li et al., J. Biol. Chem., 276: 35239-42 (2001)). Since these proteins are situated at the basolateral part of healthy cells, protein-MUC1 interactions are not believed to be significant. However, loss of polarity in human tumor cells allows the cytoplasmic tail to be exposed to the signaling proteins, and interaction can occur (see Vermeer et al., Nature, 422: 322-6 (2003)).
The MUC1-C region has been shown to act as an oncogene, leading to transformation of human cells when MUC1-C binds to β-catenin (see Li et al., Oncogene, 22: 6107-10 (2003); Raina et al., Cancer Res., 69: 5133-41 (2009); and Wei et al., Cancer Res., 67: 1853-8 (2007)). Moreover, MUC1-C transfection has been demonstrated to be sufficient to induce transformation and confer oncogenic activities previously attributed to the full-length MUC1 protein, such as increased growth rate, anchorage-independent cell growth, and resistance to chemotherapy agents (see Ren et al., Cancer Cell. 5: 163-75 (2004)). In addition, MUC1-C signaling activated by c-Src is involved in the disruption of both E-cadherin adherens junctions and integrin focal adhesions that stimulate cancer cell motility, invasion, and metastasis, thereby suggesting a possible role for MUC1-C in epithelial to mesenchymal transition (EMT) (see Hu et al., Expert Rev. Anticancer Ther., 6: 1261-71 (2006)). Overexpression of genes related to MUC1 has also been found to be highly associated with poor prognosis in patients with lung and breast cancer and with drug resistance (see Ren et al., supra; and Khodarev et al., Cancer Res., 69: 2833-7 (2009)).
Numerous clinical trials have evaluated MUC1 as a potential target for vaccine therapy of a range of human tumors. The majority of these have employed polypeptides of the VNTR region. One agonist epitope (P93L) was shown, compared to the native epitope, to enhance the generation of T cells that can also more efficiently lyse human tumor cells (see Tsang et al., Cancer Res., 10: 2139-49 (2004)). Two other potential agonist epitopes in this region were shown to enhance T-cell cytokine production, but no tumor cell killing was reported (see Mitchell et al., Cancer Immunol. Immunother., 56: 287-301 (2007)).
One method that has been shown to enhance the ability of a vaccine to be more efficacious is to make alterations in the amino acid sequence of putative T-cell epitopes, which in turn can enhance T-cell activation and specific T-cell killing of tumor cells (see Grey et al., Cancer Surv., 22: 37-49 (1995); and Terasawa et al., Clin. Cancer Res. 8: 41-53 (2002)). Not all substitutions of an amino acid of a potential cytotoxic T lymphocyte (CTL) epitope, however, will lead to an enhancer agonist epitope, and some substitutions will lead to antagonist epitopes. Moreover, the generation of a putative agonist epitope of a tumor associated antigen may well lead to enhanced T-cell activation by IFN-γ production, but will be useless unless the activated T cell will recognize the endogenous (native) epitope expressed in the context of the MHC on the surface of human tumor cells, and consequently lyse those tumor cells.
There is a desire to identify new specific cytotoxic T lymphocyte (CTL) epitopes and enhancer agonist peptides or epitopes of MUC1-C, and to develop compositions and methods that use these epitopes for the diagnosis and/or treatment of cancer.